In unsealed court filings that shed new light on what investigators think happened before, during and after the lethal explosions on April 15, prosecutors charged Tsarnaev with one count of using and conspiring to use a WMD resulting in death. The 19-year-old ethnic Chechen, a naturalized U.S. citizen, also was charged with one count of malicious destruction of property by means of an explosive device resulting in death.

If convicted on either federal charge, Tsarnaev faces the death penalty or life in prison. He also faces the possibility of state criminal charges, as well, in connection with the bombings, which killed three people and wounded more than 200. Massachusetts has no death penalty.

In a ceremony Monday, the FBI turned Boylston Street — which had been considered a crime scene — back to the city. The flag that flew over the finish line during the race was presented to Mayor Thomas M. Menino.

The street won’t be open to the public until buildings along it have been inspected for structural damage, city officials said.

The charges against Tsarnaev, filed under seal Sunday, were presented to him Monday in his room at Boston’s Beth Israel Deaconess Medical Center, where he has been listed in serious condition since his capture Friday. FBI officials said Monday that Tsarnaev was wounded in the head, neck, leg and hand after two shootouts with law enforcement officers.

“The government will always seek to elicit all the actionable intelligence and information we can from terrorist suspects taken into our custody,” said Carmen Ortiz, U.S. attorney for the District of Massachusetts.

At the same time, rejecting calls made by congressional Republicans, White House spokesman Jay Carney said the administration would not designate Tsarnaev as an enemy combatant. The designation would have permitted additional interrogation of Tsarnaev, but Carney said it was unnecessary.

“It is important to remember that since 9/11 we have used the federal court system to convict and incarcerate hundreds of terrorists,” Carney said.

The charges were unsealed in federal court in Boston shortly before the usually bustling city calmed for a moment of silence at 2:50 p.m., the approximate time of the first bomb explosion a week earlier. Church bells tolled across the city and state.

Standing on the steps of the Capitol, Massachusetts Gov. Deval Patrick bowed his head and said after the moment of silence: “God bless the people of Massachusetts. Boston Strong.”

Also, hundreds of family and friends packed a church in Medford for the funeral of bombing victim Krystle Campbell, 29.

A memorial service was held Monday night at Boston University for 23-year-old Lu Lingzi, a graduate student from China.

Fifty-one victims remained hospitalized Monday, three of them in critical condition.

In a glimmer of good news, all of the more than 180 injured people who made it to a hospital alive now seem likely to survive.

“All I feel is joy,” said Dr. George Velmahos, chief of trauma surgery at Massachusetts General Hospital, referring to his hospital’s 31 blast patients. “Whoever came in alive, stayed alive.”

Two prosecutors from the Massachusetts district’s Anti-Terrorism and National Security Unit, William Weinreb and Aloke Chakravarty, will lead the case for the Justice Department.

Tsarnaev will be represented by the federal public defender’s office in Boston.

Tsarnaev’s brother and suspected co-conspirator, Tamerlan Tsarnaev, 26, died early Friday after a gun battle with police.

The two men had carjacked a Mercedes SUV around midnight Thursday, according to an affidavit filed Monday by FBI Special Agent Daniel R. Genck.

According to Genck’s affidavit, a man was sitting in his car in Cambridge late Thursday when one of the Tsarnaev brothers approached and tapped on the passenger’s side window.

“When the victim rolled down the window, the man reached in, opened the door and entered the victim’s vehicle,” Genck recounted. “The man pointed a firearm at the victim and stated, ‘Did you hear about the Boston explosion?’ and ‘I did that.’ The man removed the magazine from his gun and showed the victim it had a bullet in it, and then reinserted the magazine. The man then stated, ‘I am serious.’”

According to the affidavit, the carjacker then forced the victim to drive to another location, where they picked up a second man, who put something in the trunk of the car. The victim further said the two men spoke to each other in a foreign language. The car’s owner said he was able to eventually escape after stops at an ATM ,a gas station and convenience store.

Video reviewed by investigators appeared to show the Tsarnaev brothers carrying large knapsacks as they walked along Boylston Street about 11 minutes before the first explosion. Further video and still photographs reportedly show one of the brothers slipping off his knapsack in front of the Forum restaurant and walking away. About 30 seconds before the first explosion, Genck recounted, video shows Dzhokhar Tsarnaev appearing to talk into his cellphone.

“A few seconds after he finishes the call, the large crowd of people around him can be seen reacting to the first explosion,” Genck recounted, while Tsarnaev “virtually alone among the individuals in front of the restaurant appears calm.”

The video then shows Tsarnaev leaving his knapsack on the ground and walking away, Genck said.